The present invention relates to a method for identifying two or more movable objects in an observation or monitoring area with an essentially horizontal light grid that has a plurality of parallel light barriers and a light barrier for practicing the method.
Light grids are used, for example, to control highly automated manufacturing systems or assembly lines, such as in the automotive industry, where workpieces are automatically fed to a workstation, worked on and then transported away. Such assembly or manufacturing lines typically have fully automated workstations (for example robot installations) and associated transportation and supply systems. These supply systems and/or the workstations where work is being performed present zones of potential danger for workers which require protection against unwanted intrusions. The desired protection can be provided with light barrier grids of the type described above.
The word “light” as used herein is not limited to visible light and is intended to encompass light of all types, including particularly all types of ultraviolet light, infrared light, as well as visible light.
Access areas and transfer locations for materials to dangerous rooms, for example robot stations or other machines, are conventionally protected by so-called muting systems. Light barriers are employed, for example, to monitor the entry of persons, and they can be temporarily deactivated or muted when material is being delivered. German patent publication DE 201 03 828 U1, for example, describes such a system. Known muting systems are costly to install and to operate, which is a disadvantage. A particular disadvantage of such systems is that under muting conditions the control function is completely blocked, and during such times the control of access to the dangerous area is deactivated. The emitters and receivers of light barrier grids of muting systems must be constructed so that no person can enter the danger zone through the space between the sensors and the muting object. The space between the sensors is therefore typically secured with swinging doors.
German patent publication DE 44 24 537 discloses a system in which a light barrier grid recognizes objects as so-called floating blanking objects by monitoring for predetermined sizes and positions of the objects. Such systems are limited to monitoring objects within certain size and position ranges. It is therefore not possible to have two or more objects of predefined but differing size and spacing (from each other) simultaneously within the light barrier grid without deactivating the system.
As a result, prior art systems of the type described above encounter the problem of differentiating between permitted objects (for example machine components) and impermissible objects (for example the legs of a person) in the area being monitored.